GENERATORS

Of these other responses, the one that is most useful in the auditory field is the vertex positive component P3 or P300 with a latency of about 300 ms (see Fig. 2.7). The P3 potential is thought to be associated with a number of cognitive processes and with the short-term memory function (Goodin et al 1978, Hillyard 1984). The scalp topography of the P3 amplitude is maximal in the central-parietal mid-line area, and it is bilaterally symmetric (Simson et al 1977). After measuring intracranial potentials, Halgren et al (1980) suggested that P3 is an endogenous potential generated in subcortical structures such as the hippocampal formation and the amygdala. Goff et al (1980), using intracranial recordings, suggested that neural origins of late-latency evoked potentials may be associated with deep-brain structures, and may not be directly generated from the cortical surface.

There are also slow direct-current potentials of cortical origin associated with expectancy; these are described by Walter (1964) as contingent negative variation (CNV), which is a pre-signal potential; it is schematically shown in Figure 2.5.